In the assembly of metal parts into a product or other component, welding is often used. For example, sheets of metal can be spot welded together, wherein the sheets are temporarily clamped together and a large current is forced through a small area of the overlapping sheets. The weld is formed by a partial melting of the metal in the small area. As a result, the weld usually does not protrude significantly above the sheet surface.
Welding is not always possible or practicable, however. In some situations, when an aluminum piece is to be joined to a steel piece, riveting is used instead of welding. For example, self-piercing rivets can be used. However, the rivet head protrudes above the flange through which it is fastened. Therefore, if another piece or surface abuts the fastened area, it must be positioned on top of or above the rivet heads.